Ultrasonic surgical instruments may be used in medical procedures to, for example, dissect or cut living organic tissue. The dissecting or cutting action is accomplished by a surgical implement at the distal end of the instrument, the surgical tip transmitting ultrasonic power to the tissue. Ultrasonic power may also be used to cauterize tissue surrounding the surgical tip, causing hemostasis by coagulating blood in the surrounding tissue.
Ultrasonic vibration is induced in the surgical tip by electrically exciting one or more piezoelectric elements in the instrument handpiece. The piezoelectric elements are excited at a resonant frequency of the surgical instrument. The fundamental resonant frequency of the surgical instruments described herein is f.sub.o and the resonant wavelength is .lambda..sub.o. .lambda..sub.o is equal to c/f.sub.o where c is the speed of sound in the instrument. The electrical excitation signal being sinusoidal in nature or having a sinusoidal component, the piezoelectric elements generate a mechanical standing wave vibration at a frequency equal to the frequency of the electrical excitation signal. Vibrations generated by the piezoelectric section are filtered through a resonator, amplified by at least one velocity transformer and transmitted to the surgical tip.
In the ultrasonic surgical instruments described herein, the hand piece, which is also known as the "Langevin Stack" or "stack", includes at least one piezoelectric section surrounded by a pair of resonators. The resonators generally comprise metal sections which abut the piezoelectric section on each side and extend a distance of approximately one quarter wave length (.lambda..sub.o /4) from the center of the piezoelectric stack. The stack, including the piezoelectric elements and the resonators, is approximately one-half wave length (.lambda..sub.o /2) long. The resonators act as a mechanical bandpass filter, filtering out or substantially reducing acoustic waves at frequencies above or below the frequency of operation and above or below harmonics of the frequency of operation. One end of the stack may be adapted to receive the cable or wire which transmits the electrical excitation signal to the piezoelectric elements. In many such instruments, the resonators are electrically grounded and the excitation signal is connected to alternating pairs of piezoelectric elements within the stack. The opposite end of the stack is generally adapted to receive either the surgical tip or a transmission rod which is adapted to transmit the acoustic signal from the stack to a surgical tip.
In surgical instruments such as the instruments described above, the acoustic waves may be amplified by inserting "velocity transformers" between the resonator and the tip. A velocity transformer amplifies the acoustic wave by reducing the cross sectional area of the instrument. Therefore, by using a transmission rod with a smaller diameter than the resonator output, the junction between the resonator and the transmission rod acts as a velocity transformer, increasing the intensity of the acoustic wave transmitted through the transmission rod. Where the tip is smaller in diameter than the transmission rod, a similar step junction at the interface between the transmission rod and the tip acts as a second velocity transformer, further increasing the intensity of the acoustic wave at the tip of the instrument. Generally, the length of the resonators and transmission rods are chosen to ensure that the velocity transformers are located at nodes in the standing wave pattern.
As stated previously, an acoustic standing wave pattern is initiated by applying an electric voltage at a frequency f to the piezoelectric elements. The application of an electrical voltage across a piezoelectric element results in the expansion or contraction of the piezoelectric element along the axis of the voltage gradient. The direction of displacement of the piezoelectric element (i.e. whether it expands or contracts) is determined by the polarity of the signal applied. Thus, when an electrical excitation a signal with a sinusoidal component at a frequency f is applied, the piezoelectric elements expand and contract in a continuous manner. Mechanical expansion and contraction of the stack itself may be avoided by using the piezoelectric elements in pairs, where the application of an electric signal of a given polarity causes one element of the pair to expand and the second element of the pair to contract.
An ultrasonic surgical instrument may be driven or excited at any of its resonant frequencies. The instrument, having a fundamental resonant frequency of f.sub.o may be driven by an excitation signal f.sub.c at any harmonic (i.e. whole integer multiple) of f.sub.o. A signal at the excitation frequency f.sub.c has a wavelength .lambda..sub.e. Since the fundamental resonant frequency of the instrument is f.sub.o, an electrical excitation signal wave which drives the piezoelectric elements at a frequency of f.sub.o will cause the surgical tip to vibrate at f.sub.o. Where the electrical excitation signal is not a clean sine wave (e.g. a square or sawtooth wave) it includes harmonics of the excitation signal and the surgical tip would be expected to vibrate at f.sub.o and at harmonics of f.sub.o. Even where the electrical excitation signal is a clean sine wave, nonlinearities in the instrument may cause the surgical tip to vibrate at f.sub.o and at harmonics of f.sub.o. In either case, the magnitude of the vibration at the surgical tip would be greatest at f.sub.o, and would be expected to be substantially smaller at harmonics of f.sub.o. In most cases, the power at the harmonic frequency would be less than 25% of the power at the fundamental frequency. Thus, in known generators, power transmitted by the surgical tip at frequencies other than the excitation frequency would be expected to be at most 25% of the power transmitted at of the excitation frequency of the instrument. The relative power in the excitation frequency and its harmonics is a function of the degree to which the excitation signal deviates from a clean sine wave. Therefore, where the excitation signal drives the piezoelectric elements at f.sub.o, substantially all the work done by the tip (e.g. cutting and/or cauterizing tissue) is being done at f.sub.o. Where the excitation signal drives the piezoelectric elements at any other excitation frequency f.sub.e, substantially all the work done by the tip is being done at f.sub.e.
Certain vibration frequencies are believed to enhance the operation of an ultrasonic instrument for a particular purpose. For example, a given frequency may be particularly beneficial when using the instrument to cut or dissect tissue, while a second, different frequency, may be beneficial when using the instrument to cauterize tissue. Therefore, driving the piezoelectric elements at both a first excitation frequency, for example, the fundamental frequency of the instrument, and at one or more harmonics of the excitation frequency could, in some applications, substantially improve the performance of the instrument. For example, where the instrument is being used to cut and coagulate tissue, the excitation signal might be selectively controlled to provide power at the fundamental frequency for cutting tissue and at a harmonic of the fundamental frequency for cauterization the tissue.
Since the application of acoustic power to tissue in the form of ultrasonic waveforms tissue may result in different beneficial effects depending upon the frequency of the ultrasonic power applied to the tissue, it would be beneficial to design an ultrasonic surgical instrument where the piezoelectric stack is excited at both a first excitation frequency, for example, fundamental frequency of the instrument and at least one harmonic of the excitation frequency. It would further be beneficial to design an ultrasonic instrument wherein the power supplied at harmonics of the first excitation frequency is substantially equal to or greater than 50% of the power supplied at the first excitation frequency.